1. Field of the Invention
The present invention generally relates to a method of making vertical contact holes through an insulator to a silicon substrate, and, more particularly, to a method of etching vertical contact holes without substrate damage caused by directional etching.
2. Description of the Related Art
In the semiconductor industry, there is a need to create vertical contact holes through an insulator to the silicon or other semiconductor substrate to minimize image size. A typical technique employs an appropriate mask positioned over an insulator to define the contact holes. This is followed by a directional etch, such as an anisotropic reactive ion etch (RIE) that etches the underlying insulator to form the vertical contact holes.
However, vertical contact processing using a directional or reactive ion etch (RIE) generally damages (etches) the underlying silicon substrate as well, since the RIE etch is not selective enough to stop on the silicon substrate. The damage--caused by chemical and/or ion bombardment attack--limits precise definition of shallow emitters, especially for bipolar technologies.
Wet etching of contact holes through the insulator is extremely selective and does not damage the silicon substrate. The wet etch, however, results in contact holes having curved edges through the insulator to the substrate, which undesirably increases the contact hole size.
In an effort to overcome such difficulties, shallow emitter technologists and designers have employed an etch stop in the vertical RIE contact process, followed by a subsequent isotropic wet etch of the etch stop. While the wet etch leaves the silicon surface undamaged, it also undercuts the etch stop in the vertical contact hole since the isotropic wet etch etches equally in all directions. This causes a problem for subsequent metal coverage and/or proper contact only to the emitter.
In light of the foregoing, there exists a need for a method of creating vertical contact holes or vias without directional etching damage to the underlying substrate and without the undercutting problem associated with isotropic wet etches.